1. Field of the Invention
This invention relates to a mold-clamping device of a blow molding machine usable when a hollow-molded product is manufactured by a blow molding process.
2. Discussion of the Background
A mold-clamping device 1' as shown in FIG. 8 is known as a conventional technique relating to a mold-clamping device of a blow molding machine to be used when a molded product is produced by a blow molding process. This mold-clamping device 1' includes a body portion 2', tie bars 3', 3' forwardly and backwardly movably supported in their horizontal postures by the body portion 2', a front platen 4' and a rear platen 5' provided on a front end portion and a rear end portion of the tie bars 3', 3', respectively, a movable platen 6' forwardly and backwardly movably disposed on the tie bars 3', 3' in such a manner as to face with the front platen 4', split mold halves 7', 8' disposed on the mutually opposing sides of the front platen 4' and the movable platen 6', and a pinion-rack mechanism 10' for guiding the movement of the rear platen 5' and the movable platen 6'.
The mold-clamping device 1' is installed such that a parison P hanging down from a cross head C of an extruding device S can be guided into a cavity of the split mold halves 7', 8'. Then, the molding is started. First, a solenoid valve 12' and one pair of servo valves 13' are controlled by a hydraulic unit 11' such that a piston rod 90' of the piston-cylinder mechanism 9' is pushed to move the movable platen 6' towards the parison P and to move the rear platen 5' in an opposite direction to the parison P. Then, the front platen 4' is caused to move towards the parison P by the backward movement of the tie bar 3' in response to the movement of the rear platen 5', so that the parison P is held between the split mold halves 7', 8'. After the finish of the blow molding, the piston rod 90' is contracted to cause the rear platen 5' and the movable platen 6' to escape from the molded product, thereby completing the molding operation.
By the way, in the above mold-clamping device 1', it is normally required to operate the hydraulic unit 11' in order to stabilize the hydraulic even when opening-and-closing operation of the split mold halves are not undergoing, and in addition, the thrust force of the cylinder-piston mechanism 9' must be maintained in its generated state in order to cope with the elevation of the pressure within the split mold halves 7', 8'. Therefore, the running cost is inevitably increased. Moreover, there are involved various problems liable to occur to hydraulic devices, such as fluctuation of operating time due to leakage of oil and change in oil temperature, dirty losses and noises during molding, indispensable space increase, etc.
Furthermore, since the tie bars 3', 3' are arranged only on the underneath with respect to the split mold halves 7', 8', it is difficult that the clamping force is uniformly applied to the abutment surfaces of the split mold halves 7', 8'. Although the front and rear platens 4', 5' are provided with clamping force adjusting bolts 14', 15' for adjusting the clamping force, it is nevertheless difficult to obtain a uniform clamping force. Therefore, the larger clamping force than necessary at the abutment surfaces of the split mold halves 7', 8' is applied to the front and rear platens 4', 5'. As a consequence, the front and rear platens 4', 5', and the movable platen 6' are unduly required to be increased in physical strength to that extent. This eventually results in a large size of the device as a whole.